The hot workability of modified 9Cr-1Mo, as a grade of heat resistant steels, is inferior to that of low-alloy steel, so the inner surface crack (ISC) easily occurs in seamless boiler tubes produced by the Mandrel Mill under improper rolling conditions. With the aid of FEM, the metal flow status during the rolling process was analyzed in 140mm 8-stand mandrel mill of Bao Steel. Both the metallographic shape and size of the ISC together with the result from the simulation show that the ISC of seamless tube forms at the elongation stage of shell. The mechanism of the ISC was discussed. With its initiation in stand No.1 and No.2 due to poor hot workability of modified 9Cr-1Mo steel, the ISC develops in subsequent passes. Based upon the mechanism devised was a special roll pass system which substantially upgraded the yield of qualified products.